1. Field of the Invention
The present invention relates to an active matrix display device. In particular, the invention relates to an active matrix display device which employs a display method of the in-plane switching mode (also called IPS mode). The invention is intended to reduce potential variation of signals (or data) to thereby lower the power consumption, and to reduce voltages applied to the switching elements that are provided for the respective pixels to thereby lower the loads of the switching elements. The invention also relates to a driving method of a capacitive-coupling-type display device such as a liquid crystal display device.
2. Description of Related Art
In a capacitive-coupling-type display device such as a liquid crystal display device, it is necessary to invert the polarity of a voltage applied to a pixel capacitor element. This operation is also called alternating. This is because if electric fields in one direction are always applied to an electro-optical material (a material whose optical property such as light transmittance, reflectance, or a refractive index varies depending on the voltage applied thereto) provided between the electrodes of a capacitor element, the material will deteriorate. It is necessary to invert the polarity of the voltage every field (or frame) or every several fields.
Among various inverting methods, there are a field (or frame) inverting scheme in which the polarity is the same over the entire display screen in each field (see FIG. 10A), and a gate line inverting scheme in which the polarity of each row is different from adjacent rows, (see FIG. 10B). The above methods can be applied to the IPS mode.
Conventionally, the polarity inversion is performed such that each pixel is supplied, from a data driver (signal driver), with a signal whose polarity is inverted. FIG. 7 shows a unit pixel of a conventional active matrix liquid crystal display device. A thin-film transistor T as a switching element is controlled by a signal (selection pulses) on a scan line Xn. In a state that a selection pulse is applied to the thin-film transistor T (on-state), a signal on a data line (signal line) Pm is supplied to a liquid crystal pixel element LC and, if necessary, to an auxiliary capacitor C connected in parallel with the pixel element. On the other hand, the potential of a common line (or common electrode) Yn is kept constant. Charge is stored in accordance with a difference between the potential supplied from the data line Pm and that of the common line Yn.
FIG. 8 shows drive signals in a display device in which such unit pixels are arranged in an N-row matrix. In FIG. 8, a clock signal (sync signal) CLK indicates a minimum operation time of the display device. Signals are generated based on the clock signal CLK. As shown in FIG. 8, selection pulses are sequentially applied to scan lines X1, X2, X3, . . . , XN−1, XN. On the other hand, potentials depending on image signals for the respective rows are applied to a data line P1. This example is directed to the field inverting scheme (FIG. 10A). For convenience of comparison, it is assumed that the image information of the fields are always the same; that is, the data of the second field is an inversion of that of the first field with respect to the reference potential (i.e., the potential of the common lines). The same relationship exists between the second and third fields.
FIG. 9 shows an example of data in the case of the gate line inverting scheme (FIG. 10B). The data for each row has opposite polarities between the first and second fields.
As described above, in the conventional active matrix liquid crystal display devices, the driver needs to generate data whose variation range is two times that of a signal required by only image information. That is, although basically it is sufficient to apply a liquid crystal with effective voltages of a 5 V, the necessity of inversion requires a variation range of 10 V, i.e., +5 V to −5 V. This increases drive voltages of the driver, and hence is the greatest obstacle to reduction in power consumption.
There is another problem because of the increase of large potential variations of data, output potential differences (i.e., selection pulse heights) of the scan driver, and power consumption therein. Further, due to large voltages applied to the active matrix circuit, the switching elements (transistors) will possibly be broken or their characteristics will possibly deteriorate.
The present invention has been made in view of the above problems, and an object of the invention is therefore to provide a device configuration and a corresponding driving method which enable necessary polarity inversion while minimizing data variations.
In the in-plane switching (IPS) mode, a display is performed by applying electric fields of which directions are parallel with a substrate surface by means of a single substrate, in contrast to the conventional liquid crystal display devices in which display is performed by applying, between the substrates, electric fields perpendicular to the substrates. Japanese Examined Patent Publication No. Sho. 63-21907 discloses the basic concept of the IPS mode in an active matrix liquid crystal display device using thin-film transistors as switching elements.
Among the inventions made by adapting the above basic concept of the IPS mode are disclosed in Japanese Unexamined Patent Publication Nos. Hei. 7-43744, Hei. 7-43716, Hei. 7-36058, Hei. 6-160878, Hei. 6-202073, Hei. 7-134301, and Hei. 6-214244. Further, Japanese Unexamined Patent Publication No. Hei. 7-72491 is directed to a case where the IPS mode is used in a passive matrix liquid crystal display device. Japanese Unexamined Patent Publication No. Hei. 7-120791 is directed to a case where the IPS mode is employed in an active matrix liquid crystal display device using thin-film diodes as switching elements.
The operation principle of the IPS mode disclosed in the above prior art references will be briefly described below with reference to FIGS. 5 and 6. FIG. 5 shows a unit pixel of an active matrix liquid crystal display device using the IPS mode. As in the case of ordinary active matrix liquid crystal display devices, data lines 11 and scan lines 12 are arranged in matrix form. In addition, common lines (also called opposed electrode lines) 13 are provided.
Conventionally, the common lines 13 are not necessary in the substrate because the opposed substrate has them. However, in the IPS mode in which the opposed substrate has no electrode, wiring lines (i.e., common lines 13) having a function equivalent to that of the above electrode need to be provided on the substrate concerned.
In the conventional IPS mode, the potential of the common lines 13 is kept at a constant value. Where the common lines 13 are formed at the same time as the scan lines 12, the former is patterned so as not intersect the latter, that is, so as to be parallel with the latter. With this structure, the common line 13 may be overlapped with a pixel electrode 14 which is formed at the same time as the data line 11, to form an auxiliary electrode C.
That is, the scan lines 12 and the common lines 13 can be formed at the same time and the data lines 11 and the pixel electrodes 14 can also be formed at the same time. A switching element (thin-film transistor, i.e., TFT) is formed as shown in FIG. 5 with a portion of the scan line 12 used as a control electrode (i.e., gate electrode). The input terminal (source) of the switching element is in contact with the data line 11 and the output terminal (drain) is in contact with one electrode (pixel electrode 14) of the pixel capacitor element. The common line 13 serves as the other electrode of the pixel capacitor element.
In FIG. 6, since the common line 13 is so formed as to be opposed to the pixel electrode 14 as described above, when a potential is given to the pixel electrode 14, electric fields indicated by arrows develop between the pixel electrode 14 and the common line 13. Where a liquid crystal is used as the electro-optical material, in the initial state liquid crystal molecules are so oriented as to form a predetermined angle with expected electric fields (state a in FIG. 6). For example, in the case of a nematic liquid crystal, the predetermined angle is 15°. When electric fields are applied, liquid crystal molecules tend to become parallel with the electric field (state b in FIG. 6). Gradation can be expressed by properly utilizing the inclination of liquid crystal molecules. The description of the operation principle of the IPS mode concludes here.
The IPS mode has a feature of a wider viewing angle than in the conventional liquid crystal display devices because the liquid crystal is oriented parallel with the substrates. However, in the above-described prior art of the IPS mode, no consideration is made of reduction in the load of the data driver; data are generated in the same manner as in the conventional cases.